The present invention relates to a technology for improving the characteristics of a semiconductor laser with an external cavity, in which intensity noise is not generated and the oscillation frequency shift or chirping associated with current modulation can be suppressed.
A remarkable improvement in the properties of a semiconductor laser has been made recently, such as reduction of threshold current, a high output power, a stabilized single lateral mode, and a stabilized single longitudinal mode. The semiconductor laser is widely applied not only to optical communication, but also to light sources for an optical sensor and an information processing system using e.g. an optical disk, and the like. There exists, however, a substantial problem in a semiconductor laser that the oscillation frequency or oscillation wavelength varies when the drive current of the semiconductor laser is modulated. The amount of oscillation frequency shift caused by the current modulation depends largely upon the semiconductor laser structure and the modulation frequency of modulation current.
FIG. 1 shows a typical example of such dependence, upon the modulation frequency fm, of the amount .DELTA..nu./.DELTA.I [Hz/mA] of oscillation frequency .DELTA..sub.o shift of a semiconductor laser per unit current, when the solitary semiconductor laser is current modulated. The characteristic shown in the figure is one example obtained from a buried hetero-structure type semiconductor laser, and the detailed description thereof may be obtained from "Direct frequency modulation in AlGaAs semiconductor lasers", by S. Kobayashi et al. IEEE J. Quantum Electron., vol. QE-18, pp. 582-595, Apr. (1982).
As seen from the example shown in FIG. 1, when a modulation current is supplied to a laser, the oscillation frequency shift is generated which takes a minimum .DELTA..nu. under the frequency of at about fm=50 MHz and .DELTA..nu./.DELTA.I=100 MHz/mA. Under modulation at a lower frequency (fm&lt;50 MHz), the oscillation frequency shift increases due to the refractive index change caused by the temperature change of an active layer of the semiconductor laser, and under modulation at a higher frequency (fm&gt;50 MHz), the oscillation frequency shift increases due to the refractive index change caused by the fluctuation of carrier density.
The oscillation frequency shift may be effectively utilized if the semiconductor laser is used as a light source for frequency modulation, however, undesirable cases may often happen for some applications. For instance, in the long-haul optical communication for transmitting digital signals, the spectral line broadening due to the semiconductor laser modulation restricts the transmission distance or the transmission bandwidth. Further, in the transmission system wherein a semiconductor laser is coupled to a multi-mode optical fiber, the oscillation frequency modulation due to current modulation causes the increase of intensity noise, distortion, and so on. In an optical fiber sensor as well, a large fluctuation of the oscillation frequency may induce some problems such as noise.
It is understood from the above description that the laser oscillation frequency shift under intensity modulation by direct current modulation of a semiconductor laser is an extensively undesirable phenomenon, and this can be considered as one of the disadvantages of the semiconductor laser. Further, the oscillation frequency shift of the semiconductor laser is a phenomenon commonly presented even for a DFB (Distributed Feedback) laser and a DBR (Distributed Bragg Reflector) laser having a built-in diffraction grating structure. Thus, it is a problem of unavoidable kind no matter how stabilized the longitudinal single mode characteristic is.
Also known in the art is a semiconductor laser utilizing light fed back thereto, that is, a so-called coupled cavity structure semiconductor laser. The laser of this kind derives from an oscillation method selecting only a single longitudinal mode of a laser, which is described in "Mode suppression and single frequency operation in gaseous optical masers", by Kogelnik et al Proc. IRE pp. 2365-pp. 2366 (1962). A similar method thereto is also applied to a semiconductor laser. For example, as disclosed in "Short-coupled-cavity InGaAsP Injection Lasers for CW and high speed longitudinal mode operation", by C. Lin et al Electron Lett. vol. 19, pp. 561 (1983), a longitudinal single mode oscillation is realised, even under a high speed digital modulation, by mounting a reflective surface contiguous to the semiconductor laser.
A longitudinal single mode is attained by making L fairly smaller than an optical path length .eta..sub.1 l of a semiconductor laser chip, wherein l is the laser chip length and .eta..sub.1 is a refractive index of an active layer of the laser, and L is the optical path length of an external cavity. In the above document, L/.eta..sub.1 l is 76/(3.6.times.83).apprxeq.1/4, and in other publications, L/.eta..sub.1 l is set within a range of 1/3 to 1/6. Generally, even if L/.eta..sub.1 l is clearly stated, an optical feedback amount is not stipulated. The reason why the optical feedback amount is not definitely stated can be explained from the concept that a single mode oscillation is generated using a coincidence mode wherein the longitudinal mode spacing of a solitary laser matches with the longitudinal mode spacing of an external cavity mounted outside of the solitary laser. Therefore, no idea is found in the above structure for suppressing the laser oscillation frequency shift caused by a drive current modulation.
Other cases with the larger L/.eta..sub.1 l have been reported. For example, as described in "Spectral characteristics of semiconductor lasers with optical feedback", L. Goldberg et al, IEEE J. Quantum Electron., vol. QE-18, pp. 555-564, Apr. (1982), it has been pointed out that the longitudinal mode characteristic is turned to be a multi-mode one as the optical feedback amount increases, and that is desirable to perform an optical feedback with a small amount of light. The resultant structure has no concept to make the optical feedback large nor does it have the concept of drive current modulation. Therefore, no concept is found also in this structure for suppressing the laser oscillation frequency shift caused by a drive current modulation. The reason why the optical feedback amount of the semiconductor laser is generally indefinite, and the reason why it is reported that the smaller the optical feedback amount is, the more preferable, can be related to intensity noise generated when reflected light is fed back from the outside to the semiconductor laser.
It is known in the art that as reflected light is fed back to a semiconductor laser, the characteristics thereof are greatly influenced and it exhibits complicated characteristics. The feedback of reflected light induces such characteristics as increase of intensity noise of the semiconductor laser, change of longitudinal mode spectrum, non-linearity of current-light output characteristics, and the like, which represent obstacles for attainment of stabilization and high quality of the semiconductor laser. More particularly, the increase of intensity noise becomes a difficult problem in practice in the application where the semiconductor laser is used as a light source for optical communication in an analog transmission system or a coherent transmission system. In addition, the increase of intensity noise due to re-injection of reflected light has become a significant problem in the application where that the semiconductor laser is used as a light source for an information processing system employing e.g. an optical disk or a light source for an optical sensor.
It is considered that a semiconductor laser provided with an outside reflection surface is generally subject to a multi-mode oscillation as increase in the optical feedback amount from the reflection surface, and that intensity noise also increases. Numerically it is considered that multi-mode oscillation starts as the optical feedback amount exceeds more than 1 percent.
Thus, in order to reduce the influence of intensity noise increase due to feedback light to the semiconductor laser, measues such as the following are used:
(1) Insertion of optical isolator, PA1 (2) Reflection coating of laser cleaved facets, PA1 (3) Anti-Reflection coating of laser cleaved facets, PA1 (4) Superposition of high frequency modulated signal, and PA1 (5) Self-pulsation effect.
The former two (1) and (2) are methods for retaining such laser characteristics as a single longitudinal mode in longitudinal mode characteristics, and the latter three (3), (4), and (5) are methods for making the longitudinal mode characteristics those of a complete multi-mode. In any case it is considered better for the reflected light not to be returned back to the semiconductor laser.